Backing layers that can be used to magnetically record, and subsequently, to retrieve, information require excellent lubrication at their surface. Contact between the magnetic head and the outermost surface of the backing layers of the film is necessary, however, this imposes a great amount of stress to the backing layers and may result in rupture of the layer, and in loss of signal. Good lubrication allows for multiple transports of the film through various magnetic head-containing equipment. The lubricant must also remain effective after the film has been run through photographic processing solutions.
In general, the transparent magnetic layer and the lubricating layer are applied in separate coating steps. This reduces the manufacturing efficiency of the product by requiring several coating stations. Alternately, the lubricious material can be added directly to the transparent magnetic layer. However, this typically weakens the layer and can result in premature rupture of the layer and loss of signal or recorded information. Furthermore, when the lubricant is added directly into the magnetic layer and coated and dried, the lubricant will be distributed throughout the magnetic layer and may not reside primarily at the surface where it is required for optimal performance.
Photographic elements containing transparent magnetic oxide coatings on the side opposite the photographic emulsions have been well-documented. The need for lubricating layers on the magnetic oxide coatings has also been well-described. A variety of types of lubricants have been disclosed including fatty acids, fatty acid esters, silicones, waxes, etc. Typically these layers have been applied by first coating a solution of the magnetic oxide layer onto a support using a bead coating technique. The coating is then dried and a lubricant layer is then coated over the magnetic layer using a similar technique. Alternatively, the lubricant can be added to the magnetic oxide coating solution such that both the magnetics and lubricant are coated simultaneously. This is advantageous because less coating stations are required, likely reducing waste and simplifying the production. Unfortunately, in order for the lubricant to be effective it must primarily reside at the uppermost surface of the dry coating. When the lubricant is added to the magnetic oxide solution, it is difficult for the lube to get to the surface. As the solution dries rapidly, the polymeric binder for the magnetic oxide will vitrify or solidify, which retards the mobility of the lubricant. Additionally, the lubricant may also go to the support/magnetics interface instead of the desired magnetics/air interface. The result is an improperly lubricated surface, or a coating with a high coefficient of friction.
Another drawback of adding the lubricant directly to the magnetics layer is that phase separation can occur resulting in a translucent or opaque film. The lubricant can destabilize the magnetics dispersion, resulting in flocculation of the particles. Also, the lubricant may not be compatible with the magnetics binder, which can lead to gross phase separation and loss of optical transparency. It is desired to have the lubricant phase separate and migrate to the interface, without the loss of optical transparency. Obviously a very selective phase separation is desired and is difficult to control. Alternatively, the lubricant may not be soluble, or dispersible in the same solvents as are needed for the components of the transparent magnetic layer.
Multilayer coatings of transparent magnetic layers, including simultaneously coating multiple magnetic oxide containing layers, and simultaneously coating antistat and magnetic oxide containing layers are described in EP 0537778A1 and EP 0565870A1. However, these applications do not describe simultaneously coating a lubricant layer and the magnetic oxide containing layer.
The use of fatty acid ester derivatives as lubricants for transparent magnetic oxide-containing packages is discussed in the following references. U.S. Pat. No. 5,294,525 teaches the use of many stearates as lubricants within the transparent magnetic layer, however, there is no mention of stearamide. EP 0 476 535 A1, 1991 (APP number 91115547.1) teaches a variety of lubricants. U.S. Pat. No. 4,275,146 claims the use of higher fatty acid amides containing at least one double bond for a photographic element; however there is no discussion of a transparent magnetic layer. U.S. Pat. No. 5,336,589 lists typical slip agents, including stearamide and erucamide. Methods of introducing the slip were to dissolve it in the layer, or to add it by spray coating, dip coating, bar coating, or spin coating.
In this invention is described a means of co-coating the magnetics and lubricant layers but without the problems described above. In this case the well-known simultaneous coating technique, slide coating, is used. However, as will be shown in the examples, simply simultaneously coating the two solutions does not ensure success. The present invention provides low coefficient of friction and durability for excellent performance under a magnetic head. The combination of the magnetic layer and the lubricant is transparent and does not interfere with the transmission of light through the photographic element.